Method of powder rolling nickeliron-cobalt alloys

ABSTRACT

THE ALLOY 29 NI, 17 CO, BALANCE IRON IS PREPARED AS A FINE POWDER IN THE FULLY ANNEALED CONDITION AND IS ROLLED WITHOUT ADDED BINDERS INTO A GREEN STRIP OF ABOUT 80% DENSITY. AFTER SINTERING AT ABOUT 1800*-1900*F. FOR AN HOUR, IT IS REDUCED BY ROLLING ABOUT 18 TO 20%, AFTER WHICH IT IS RESINTERED. BY CONTROLLING COIL AND ROLL DIAMETERS, EDGE BREAKING, CURLING AND CRACKING ARE PREVENTED. A UNIFORM FOIL AS THIN AS 0.004 IN. CAN BE PREPARED IN THIS WAY. THE PRESENT INVENTION IS AN IMPROVEMENT OVER THE METHOD DESCRIBED AND CLAIMED IN MY PREVIOUS U.S. PAT. NO. 3,396015, ISSUED AUG. 6 1968.

United States Patent Int. Cl. B22f 1/00 US. Cl. 75-214 9 Claims ABSTRACT OFI'THEVDISCLOSURE The alloy 29 Ni, 17 Co, balance iron is prepared as a fine powder in the fully annealed condition and is rolled without added binders into a green strip of about 80% density. After sintering at about l800-1900 F. for an hour, it is reduced by rolling about 18 to 20% after which it is resintered. By controlling coil and roll diameters, edge breaking, curling and cracking are prevented. A uniform foil as thin as 0.004 in. can be prepared in this way. The present invention is an improvement over the method described and claimed in my previous U.S. Pat. No. 3,396,015, issued Aug. 6, 1968.

BACKGROUND OF THE INVENTION Field of the invention The invention relates generally to the powder rolling of metal sheets and foils and, more particularly, to the production of foils comprised of the alloy 29 Ni, 17 C0, balance iron. This alloy is commonly referred to by the trademark Kovar, other trademarks associated therewith include Sealvar, Milo-K, and Therlo. Alloys with similar, but not identical, compositions include Fernico (28 Ni, 18 C) and Rodar (0.30 Mn).

These alloys are particularly useful in electrical and electronic applications, because of their low coefiicients of expansion over a wide range of temperatures. This suits them for use in glass-to-metal seals, where Corning glass 7055 and 7040 is used, the latter matching the alloys expansion characteristics. The alloy is stamped, drawn or otherwise formed into a large variety of shapes which are used either as an insert between metal and glass of ceramic portions of a package, or as a metallic part of the package itself.

Prior art The present invention is an improvement over the above-noted prior patent, which is obviously the most relevant prior art. In the previous process, Kovar powder is rolled in the annealed condition and without added binders, into a green strip of about 80% of theoretical density. Thereafter, the green strip is sintered at 2000 F. or higher for at least two hours. The sintered strip is then reduced by rolling 30 to 50%, and the sintered roll strip is resintered, again at 2000 F. or higher, for at least two hours. The sintering conditions were considered essential for the production of a sound metallurgical structure.

As used herein, the term nickel-cobalt-iron electrical alloy is defined to include all of the following alloy compositions and minor variations thereof. This is thought to be an adequate designation, as these alloys are be- 3,667,942 Patented June 6, 1972 lieved to be the only alloys in the nickel-oobalt iron ternary system in commercial use.

Alloy name Fe Ni Co 0 Mn NOTE.Kovar is the registered trademark of Westinghouse 00., Therlo is the registered trademark of Driver-Harris 00., Rodar is the registered trademark of W. B. Driver 00., and Fernieo is the registered trademark of General Electric Co.

OBI ECT S AND BRIEF DESCRIPTION OF THE INVENTION It is a general object of the present invention to provide a method, of powder rolling of nickel-cobalt-iron electrical alloys into sheet and foil products.

Another object of the present invention is to provide an improved method of preparing sheet and foil products of nickel-cobalt-iron electrical alloys which is more economical than previous methods, specifically that of US. Pat. -No. 3,396,015.

A still further object of the invention is to provide an improved method of powder rolling nickel-cobalt-iron electrical alloys that is capable of producing thin foils of the order of 0.004 in. thick.

Yet another object of the invention is to provide an improved method of powder rolling nickel-cobalt-iron electrical alloys into sheets and foils having at least as good physical properties as conventionally prepared materials of similar compositions.

Various other objects and advantages of the invention will become clear from the following description of the novel method embodied therein, and the novel features will be particularly pointed out in connection with the appended claims.

The present invention employs the steps of powder rolling, sintering, reduction rolling and resintering that are common to most powder rolling processes. In addition thereto, the present invention involves a powder preparation step, control of the powder rolling to produce a green strip having :5 of theoretical density, specific operating parameters for sintering, reduction rolling and resintering, and limitations on roll diameter and coil diameter necessary to produce a superior product.

DESCRIPTION OF EMBODIMENTS In essence, the present invention improves on the prior patented process through change in the sintering and rolling parameters. Previously, it was thought that sintering at 2000 F. or higher for at least two hours was essential for each sintering step, and that the intermediate rolling had to be in the range of 30 to 50%, to produce a metallurgically sound product. It has now been discovered that this is not true. In particular, it has been found that an equally good product can be produced by carrying out the sintering steps for one hour at a temperature in the range of 1800 to 1900 F., provided that reduction during the intermediate rolling step is limited to about 18 to 20%. The sintering temperature can be higher, of course, but this serves no useful purpose, it was discovered. By car-rying out the present invention as herein described, the production capacity of the sintering furnaces is effectively doubled. Fuel costs are further reduced, because of the lower temperatures employed. On a unit cost basis, the product is significantly more economic to produce.

Other aspects of the method are similar to the previously patented method and are described below.

The first step in the process is preparation of the alloy powder. The preferred starting materials is a -1=00 mesh powder with individual particles being spherical in shape. This has been found adequate for producing a 0.004 in. foil, but a thinner foil would require a smaller particle size powder, inasmuch as a 0.004 in. foil is close to having a single particle thickness at 100 mesh.

The powder must be fully annealed to the soft condition. This can be accomplished by heating at 1300 to 1350 F. for about 1 to 3 hours. Those skilled in the art will appreciate that all heating steps during the process must be carried out in a neutral or reducing atmosphere to prevent oxidation. Forming gas is satisfactory. The anneal in a reducing atmosphere removes any surface oxides in addition to rendering the material in the soft or fully annealed condition. Unless the anneal is carried out with the particles suspended in a gas stream or the like, there is bound to be some agglomeration of the particles, and it is necessary that the powder again be brought to l mesh. Any suitable milling apparatus that will not contaminate the powder can be used for this purpose. The powder is ready for rolling when it has a Hall flow rate of 15 to 30 seconds. The Hall flow meter determines both apparent density and flow rate; a 50 gram sample of the powder is passed through a funnel having an orifice in. in diameter and A; in. deep. The flow rate is the time the sample takes to pass therethrough (see ASTM standards B212 and B213).

In carrying out powder rolling in accordance with the invention, no binders are used; the powder is poured into the rolls without any additives or moisture whatsoever. Apparatus for powder rolling is known in the art and need not be described in detail herein. A rolling mill having the axes of the rolls in a horizontal plane is provided, and width of the green strip is controlled by metal guides which fit down over the rolls and somewhat into the nip. The powder feeds from a hopper with sufiicient static head to assure continuous flow.

The most important parameter to control during rolling is the density of the green strip, which should be about 80%. If density is too low, of course, the strip will not have structural integrity and will fall apart. It has been found that strip produced at high densities is likely to be uneven in density, which affects the quality of the finished product. While it is difiicult to quantify the allowable variations in green strip density with exactness, a variation of :5% over the desired 80% figure is considered tolerable. This is controlled by adjusting the roll gap and roll speed.

The minimum and maximum green strip thickness that can be obtained while still achieving the required density are not known. Satisfactory green strips having thicknesses of from 0.03 in. to about 0.06 in. have been produced. It is doubted that a good product could be produced above about 0.1 in. or below 0.01 in.

The green strip is sintered for at least an hour at 1800 F. or higher. As noted above, higher temperatures than 1800 to 1900 F. and longer times could be employed, but such conditions serve no useful purpose.

While the speed with which the strip can be produced is not very high, the length of the sintering operation makes it impractical to sinter on a continuous basis. Thus, the green strip must either be cut into pieces which will fit the sintering furnace, or it must be coiled. It has been determined that green strip of the proper density can in fact be coiled, provided that the coil radius is at least 200 times the thickness of the green strip. For example, a 0.04 in. green strip can be wound on a 16 in. diameter coil without breakage. It is preferred that there be some loose alloy powder on the surface of the strip during sintering, as this helps prevent sticking. The coiled strip should be placed on a perforated plate during sintering so that any of this loose powder which falls off during the operation does not itself stick or cause the coil to stick.

In most powder rolling operations the sintered strip is hot rolled to produce densification. With the present invention this has not been found necessary. A single pass reduction of about 18 to 20% is carried out on conventional equipment. To produce a superior product, it is preferred that reduction rolling be carried out in a mill having rolls that have a diameter about 100 times the green strip thickness (i.e. 4 inches for a 0.04 in. green strip). This gives good protection against curling and edge cracking. Larger rolls can cause snaking camber due to lateral restriction in the roll throat. Smaller rolls can cause excessive waving due'to density differences in the strip.

After rolling, the strip is again sintered for about an hour at 1800" to 1900 F. The same precautions against sticking should be observed as in the first sintering step.

' It will be appreciated that the strip at this stage is much stronger than the green strip, and no particular handling precautions need be taken. This second sinter results in diffusion bonding between regions of previous voids. At this point, the strip is substantially 100% of theoretical density.

From this point on, processing of the strip to produce the final product is done by conventional methods. For example, if the sintered strip is 0.02. in. thick, it may be rolled in two stages with an intermediate anneal down to a 0.0085 in. final thickness. Such a finished strip has been successfully drawn into TO-l8 headers which were sealed to approriate bases. The seals were good and met all specifications.

As noted hereinabave, foil and sheet products made in accordance with the method of the invention exhibit physical properties at least as good and in some respects better than similar products produced by conventional techniques. This is illustrated in the following tabulation, wherein published data for Kovar is compared with data on two pieces of powder-rolled material, one produced by the method of US. Pat. No. 3,396,015, and one produced by the method of the present invention.

It will be understood that various changes in the details, steps, materials, compositions and arrangements of parts may be made by those skilled in the art within the principle and scope of the invention as defined in the appended claims and their equivalents.

What is claimed is:

1. In a method for producing thin sheets of nickelcobalt-iron electrical alloys by powder-rolling of said alloy followed by sintering, thickness reduction and re-sintering, the improvements comprising:

(a) providing said alloy in powder form with a -100 mesh particle size and in the fully annealed condition; (b) compacting said powder in a rolling mill to form a green strip having about of theoretical density;

(c) sintering said green strip under non-oxidizing conditions for about one hour at a temperature of at least about 1800 F.

(d) reducing the thickness of the sintered strip by about 18% to 20%; and

(e) sintering the reduced strip again under non-oxidizing conditions for about one hour at a temperature of at least about 1800 F.

2. The method as claimed in claim 1, wherein the powder provided in step (a) has a Hall fiow rate of about 15 to 30 seconds.

3. The method as claimed in claim 1, wherein the powder provided in step (a) is annealed at from 1300 to 1350 F. for about 1 to 3 hours in a non-oxidizing atmosphere.

4. The method as claimed in claim 1, wherein the density of said green strip is between 75% and 85% of theoretical.

S. The method as claimed in claim 1, wherein the thickness of said green strip is between 0.01 and 0.1 inch.

6. The method as claimed in claim 5, wherein the green strip is wound in a coil having a radius of at least 200 times the green strip thickness prior to step (c).

7. The method as claimed in claim 5, wherein step (d) is carried out in a rolling mill, and the rolls of said mill have a diameter of about 100 times the green strip thickness.

8. The method as claimed in claim 1, wherein said alloy has a nominal composition, by weight, of 29% nickel, 17% cobalt and the balance iron.

9. In a method for producing foils of an alloy having a nominal composition, by weight, of 29% nickel, 17% cobalt and the balance iron, from a powder of said alloy by powder rolling, sintering, thickness reduction and resintering, the improvements comprising:

(a) annealing said powder at about 1300 to 1350 F. for from about 1 to 3 hours in a reducing atmosphere;

(b) size reducing the annealed powder until it has a Hall flow rate of about 15 to 30 seconds;

(c) compacting said powder in a powder rolling mill,

the absence of any added binders, to form a green strip having a density, as compared to theoretical density, of :5%, said green strip having a thickness of between 0.01 inch and 0.1 inch;

((1) winding said green strip in a coil having a radius of at least 200 times the green strip thickness;

(e) sintering the coiled strip in a non-oxidizing atmosphere for about one hour at a temperature between about 1800 F. and 1900 F.;

(f) rolling the sintered strip in a rolling mill to reduce its thickness by about 18 to 20%, thereby densifying it, the roll diameter being about times the green strip thickness;

(g) sintering the reduced strip in a non-oxidizing atmosphere for about one hour, at a temperature between about 1800 F. and 1900 F.; and

(h) forming the strip produced in step (g) into a foil of desired thickness.

References Cited UNITED STATES PATENTS 3,396,015 8/1968 La Plante 75214 2,827,407 3/1958 Carlson et a1. 75224 X 2,842,471 7/1958 Koehler 75221 X 3,331,684 7/1967 Storchheim 75214 X BENJAMIN R. PADGETT, Primary Examiner R. E. SCHAFER, Assistant Examiner U.S. Cl. X.R. 7522l, 224 

